Fibrous papers for special applications, such as tea bag paper or fibrous paper for reinforcing food casings based on regenerated cellulose must first be made wetfast. This is achieved, for example by treatment with dilute viscose solution, as disclosed in U.S. Pat. No. 3,135,613. Food casings, in particular sausage casings, based on regenerated cellulose having a fibrous paper insert (generally termed cellulose fiber skins) are generally produced by coating with viscose. Viscose, however, is strongly alkaline and to a considerable extent redissolves the cellulose hydrate which originates from the dilute viscose solution and bonds the fibers to one another. The strength of the fibrous paper therefore decreases on coating with viscose by up to 50% or even more.
This disadvantage may be avoided according to GB 1 091 105 if, as early as during papermaking, a synthetic resin is mixed with the fibers. Synthetic resins which are preferably used are polyethyleneimines and polyamine-polyamide-epichlorohydrin resins.
A still more effective bonding of the fibrous paper may be achieved if, as binder, use is made of a mixture of polyamine-polyamide-epichlorohydrin resin and polyacrylamide (U.S. Pat. No. 3,484,256). However, fibrous papers bonded with polyamine-polyamide-epichlorohydrin resins are not thermostable. If they have been heated too greatly, which can easily occur, in particular on drying, the resin binding becomes brittle and breaks.
Compared with a fibrous paper bonded with dilute viscose, a fibrous paper which is more stable on exposure to strongly alkaline coating viscose is that which is bonded with NMMO-cellulose (DE 196 37 621). For bonding the fibers, in this case, use is made of cellulose dissolved in aqueous N-methylmorpholine-N-oxide (NMMO). The bonding is still greater if during production of the fibrous paper a polyamine-polyamide-epichlorohydrin resin is added.
Finally, a fibrous paper is also known which, in addition to cellulosic fibers such as hemp fibers, also contains synthetic fibers, in particular those made of polyamide, polyester or polyolefin (WO 00/40092). The fibers in this paper are bonded with regenerated cellulose and/or synthetic resin.
A two-layer nonwoven material is further known having a first nonwoven layer of thermoplastic polymer fibers or filaments and a second nonwoven layer of cellulose fibers or a mixture of cellulose fibers and synthetic fibers (WO 01/49914). The polymer fibers or filaments of the first layer are bonded to one another by calendering at a temperature below their softening temperature. The fibers or filaments of the two layers are bonded to one another in particular by what is termed hydroentanglement. The two nonwoven layers each have a weight of about 5 to 100 g/m2. The two-layer nonwoven material is used, for example for molded parts in automobile construction or as a wiping cloth.
WO 02/02871 relates to a nonwoven having a weight of about 10 to 50 g/m2, from which, in particular, tea bags may be produced. It contains fibers of cellulose and polylactic acid. The polylactic acid fibers are thermoplastic and heat sealable so that two layers of the nonwoven may be bonded to one another by a sealed seam under the action of pressure and heat. The production of tea bags is arranged particularly simply by this means.
The fibrous paper according to GB-A 2 324 615 contains cellulose fibers, principally hemp fibers, and if appropriate fibers made of synthetic polymers. The fibers are bonded therein using a mixture of various resins. The paper is processed in particular to give heat sensitive stencil paper.
For food casings, in particular sausage casings, customarily use is made of fibrous papers having a weight of 15 g/m2, 17 g/m2, 19 g/m2, 21 g/m2, 23.7 g/m2 or 25.4 g/m2, according to the caliber of the casing.
The previously produced cellulose fiber skins generally contain 33 to 55 g/m2 of cellulose hydrate and in addition customarily about 15 to 30 g/m2 of glycerol which serves as a secondary plasticizer. The water content of the casings is generally about 8 to 10% by weight. Its total weight is generally, according to caliber, about 75 to 115 g/m2. They are thus relatively heavy and stiff and can only be handled with difficulty, which is disadvantageous, in particular in final processing (especially on shirring the fiber skins to form shirred sticks).
There have already been attempts in many ways to achieve lighter fiber skins. For this use was made of a particularly light fibrous paper having a weight of less than 15 g/m2 which was then coated with the customary amount of viscose (EP-A-0 460 146). However, this measure led to products having impaired mechanical properties. As an alternative, fiber skins were produced using standard fibrous paper and a reduced amount of viscose with simultaneously increased spinning rate (EP-A-0 733 306). It was found in this case that the strength of these fiber skins increased relatively, but they have not to date been able to establish themselves on the market.
Despite all the efforts, there is to date no fibrous paper available which is suitable for producing cellulose fiber skins, being stable to hydrolytic attack of the coating viscose and simultaneously insensitive to thermal stress. The coating viscose must in addition be able to penetrate well any newly produced fibrous paper. The fibrous paper must finally bond firmly to the regenerated cellulose.